1. Field of the Invention:
This invention pertains to the art of thermally insulated hub arrangements for large centrifugal fans of the type used for moving hot gases such as an induced draft fan handling combustion gases in a power plant.
2. Description of the Prior Art:
It has been and is conventional for centrifugal power fans handling high temperature gases in a power plant to have wheel hubs which are shrunk fit on to the rotary shaft driving the wheel. This design has been satisfactory for the majority of applications where the operating conditions are such that the rate of temperature change to which the wheel is exposed is kept within allowable limits. However, with the advent of swing-loaded power plants in the last few years, as well as the increased possibility of upset conditions, power fans are now required to withstand high gas temperature transient conditions much more frequently than in the past. Such thermal transients can cause a high rate of heat influx into the fan's rotor parts and, by nature of the configuration and location of the various rotor components relative to each other and the airstream, the influx of heat can cause a significant temperature differential between the wheel hub and the shaft resulting in a reduction of the interference fit between hub and shaft. If such a reduction is severe enough, the hub may become sufficiently loose on the shaft to result in axial shifting of the wheel along the shaft, or in unstable conditions resulting in rotor vibrations, or in damage to the mating surfaces at the hub-shaft interface, all of which may cause sufficient damage to the fan to require shutdown of the fan and accordingly that part of the power plant.
Solving the problem of a heat shrunk hub coming loose due to temperature transients by a hub insulating arrangement has been recognized in the prior art as in U.S. Pat. No. 2,836,348. That patent discloses an arrangement in which a can-like casing filled with a heat insulating material is placed around a hub, and the rim of the casing is spot welded, according to the patent, to blade supporting bars which are welded to the hubs and which support the blades at the radially outer portion of the bars. We believe the disclosed arrangement would perform satisfactorily with the smaller range of large centrifugal fans, such as those which have a shaft of about 12 inches in diameter more or less. However, the particular arrangement disclosed in that patent is considered to have some deficiencies with respect to an arrangement according to our invention, particularly if the arrangement were to be applied to fans having larger shaft diameters, such as up to, say, 32 inches (0.81 m). Since the disclosed casings are attached solely to the bars at the rims of the casing, the expansion of the bar could lift the casings outwardly so that any clearance opening adjacent the rotary shaft is enlarged with such expansion. Further, the radial dimensions of the casings are such that if the casings were applied to relatively larger diameter shafts, the centrifugal forces created by the rotation could create a problem of unbalance and instability of the casings which would also tend to open up the clearance in the vicinity of the shaft.
Accordingly, it is the aim of our invention to provide an improved heat shielding and thermal insulating arrangement, and to provide one which is applicable to a range of larger size fan wheels, and which is relatively inexpensive and can readily be applied either during the original manufacture of the wheel or as a field fix.